ИСТИНА

We first discovered fluids with the high gold concentrations upon the study of the gold mineralization of the Baltic Shield deep zones. Mineralization was discovered in the deepest man-made hole on Earth, the Kola Superdeep drillhole (SG-3, 12262 m), drilled to provide information about rocks and processes in the deep zones of the continental crust of the Baltic Shield (Kola, 1998). The borehole was drilled in the Pechenga ore district of the Russian Federation where there is a greenstone belt of Proterozoic age with gold mineralization at the present day surface. In drillcore from 9500-11000 m, a gold-bearing region with a vertical extension of about 1500 m was found, in which the gold concentration varied from 0.01 to 6.7 ppm (INAA analyses). Gold, with up to 26 % Ag, is present as small flakes (up to 10 microns) and irregularly shaped grains located in biotite, hornblende, plagioclase, and quartz. Fluid inclusions found in quartz veins between 9052 to 10744 m (sample numbers correspond to their depth) can be divided into four types (Figure 1): 1) gas inclusions of dense CO2 - type 1, 2) vapor-liquid two-phase aqueous inclusions - type 2, 3) three-phase inclusions with NaCl daughter crystals - type 3, and 4) CO2-aqueous inclusions - type 4. The LA-ICP-MS data, revealed the presence of high concentrations of gold in individual fluid inclusions from all 4 types between depths of 9500 and 11000 m (Figure 2). Gold concentration in type 1 fluid inclusions varies from 0.7 to 326 ppm (average 56 ppm, n=64), in type 2 from 4.8 to 691 ppm (average 261 ppm, n=15), in type 3 from 3.0 to 6483 ppm (average 754 ppm, n=57), and in type 4 inclusions from 5 to 8081 ppm (average 919 ppm, n=38). The ablation profile, for number of fluid inclusions in quartz (Figure 2), shows that the appearance of the gold signal during the ablation coincides with the appearance of the K and Na signals from the inclusion fluids. The presence of gold was not detected during the ablation of quartz around type 1 to 3 fluid inclusions, which indicates that gold is located only inside the fluid inclusions. The gold signal is not the continuous smooth asymmetric shape from elements in solution, but is a series of spikes indicative of particles. Type 4 CO2 inclusions are an exception as no significant signal for Na or K is present. During ablation of the enclosing quartz, gold was found in concentrations ranging from 4 to 52200 ppm. However, no gold particles were detected in this quartz, either by optical microscope or by SEM. The spikey gold signal can indicate the occurrence of gold in the fluid as ultra-fine particles. The definitive proof of the presence of gold nanoparticles in the fluid inclusions was obtained from recording the confocal UV-Vis absorption spectra in different areas of fluid inclusions. For the type 3 and 4 inclusions, spectra recorded near the bubble/solution interface exhibit a pronounced band in the region of about 500 nm. This band is characteristic for plasmonic absorbance by spherical gold nanoparticles of 18-20 nm in diameter (Link and El-Sayed, 2000). The presence in some cases the additional red-shifted band at 610-630 nm can be attributed to aggregates of the nanoparticles. The carbon isotope values of these inclusions, δ13CO2 values of -5.3 to -5.4, corresponds to values that would be consistent with a juvenile source of CO2 (Hoefs, 2009). Therefore, we envisage the presence of a stream of juvenile, predominantly CO2 fluid rising from deep in the earth. The discovery of gold nanoparticles in such fluids at high concentrations, requires a new look at the scale of the migration of gold and the ratio of the masse of the fluid relative to the metal carried by it.